The strong growth in demand for portable consumer electronics is driving the need for high-capacity storage devices. Non-volatile semiconductor memory devices, such as flash memory storage cards, are becoming widely used to meet the ever-growing demands on digital information storage and exchange. Their portability, versatility and rugged design, along with their high reliability and large capacity, have made such memory devices ideal for use in a wide variety of electronic devices, including for example digital cameras, digital music players, video game consoles, PDAs and cellular telephones.
While many varied packaging configurations are known, flash memory storage cards may in general be fabricated as system-in-a-package (SiP) or multichip modules (MCM), where a plurality of die are mounted and interconnected on a small footprint substrate. The substrate may in general include a rigid, dielectric base having a conductive layer etched on one or both sides. Electrical connections are formed between the die and the conductive layer(s), and the conductive layer(s) provide an electric lead structure for connection of the die to a host device. Once electrical connections between the die and substrate are made, the assembly is then typically encased in a molding compound which provides a protective package.
A cross-sectional side view and a top view of a conventional semiconductor package 20 are shown in FIGS. 1 and 2 (without molding compound in FIG. 2). Typical packages include a plurality of semiconductor dies, such as flash memory die 22 and controller die 24, affixed to a substrate 26. A plurality of die bond pads 28 may be formed on the semiconductor dies 22, 24 during the die fabrication process. Similarly, a plurality of contact pads 30 may be formed on the substrate 26. Die 22 may be affixed to the substrate 26, and then die 24 may be mounted on die 22. All dies may then be electrically coupled to the substrate by affixing wire bonds 32 between respective die bond pad 28 and contact pad 30 pairs. Once all electrical connections are made, the dies and wire bonds may be encapsulated in a molding compound 34 to seal the package and protect the dies and wire bonds.
In order to most efficiently use package footprint, it is known to stack semiconductor die on top of each other, either completely overlapping each other, or with an offset as shown in FIGS. 1 and 2. In an offset configuration, a die is stacked on top of another die so that the bond pads of the lower die are left exposed. An offset configuration provides an advantage of convenient access of the bond pads on each of the semiconductor die in the stack. While two memory dies are shown in the stack in FIG. 1, it is known to provide more memory dies in the stack, such as for example four or eight memory dies.
In order to increase memory capacity in semiconductor packages while maintaining or reducing the overall size of the package, the size of the memory die has become large compared to the overall size of the package. As such, it is common for the footprint of the memory die to be almost as large as the footprint of the substrate.
The controller die 24 is generally smaller than the memory die 22. Accordingly, the controller die 24 is conventionally placed at the top of the memory die stack. This configuration has certain drawbacks. For example, it is difficult to form a large number of wire bonds from the die bond pads on the controller die down to the substrate. It is known to provide an interposer or redistribution layer beneath the controller die so that wire bonds are made from the controller die to the interposer, and then from the interposer down to the substrate. Moreover, the relatively long length of the wire bonds from the controller die to the substrate slows down operation of the semiconductor device. It is known to mount the controller die directly on top of the substrate, but this then presents difficulties in mounting the larger memory die on top of the controller die.